Inonizing radiation induces mutation and chromosome aberrations in the irradiated cells. However, the frequency of chromosome aberrations induced by high-dose X-ray irradiation is diminished in cells exposed to low-dose pre-irradiation. The radio-adaptive response induced by low-dose radiation has been previously reported. These studies usually choose V79 cells or human peripheral lymphocytes as test materials. However, V79 cells are no longer normal diploid cells and in the case of human peripheral lymphocytes, artificial blastoid transformation by a mitogen is necessary for the experiments. In order to observe the radio-adaptive response of normal cells under natural conditions, we elected to study human embryonic fibroblasts as cells in the normal cell cycle under natural conditions. We investigated the dose dependency of the frequency of chromosome aberrations induced by radiation, conditions required to induce the radio-adaptive response, and the relationship between radio-adaptive response and protein production system. As a result, the number of chromosome aberrations significantly increased with high dose irradiation as a function of dose. When a conditioning irradiation of 10 cGy was given to cells 4 hours before the irradiation of 150 cGy, the number of chromosome aberrations significantly decreased. However, the induction of this radio-adaptive response was inhibited by treatment with 1 microgram/ml of cycloheximide during the 2 to 4 hour period after conditioning irradiation. These findings possibly suggest that the radio-adaptive response can be induced by pre-irradiation with about 10 cGy and that X-ray induced protein influences the induction of radio-adaptive response.